1. Technical Field
The present invention relates to a perpendicular magnetic recording medium mounted in various kinds of magnetic recording devices. More particularly, the invention relates to a perpendicular magnetic recording medium mounted in a hard disc drive (HDD) used as an external storage device of a computer, audiovisual (AV) instrument, or the like.
2. Related Art
Since 1997, the recording density of a hard disk drive (HDD) has increased sharply at an annual rate of 60 to 100%. As a result of this kind of remarkable growth, an increase in density with the in-plane recording method used to date is nearing its limit. Given such circumstances, attention has focused in recent years on a perpendicular recording method whereby an increase in density is possible, and research into and development of the method has been vigorously pursued. Then, the long-awaited commercialization of an HDD employing the perpendicular recording method was started in 2005.
A perpendicular magnetic recording medium is configured mainly of a magnetic recording layer of a hard magnetic material, a underlayer for orienting the magnetic recording layer in an intended direction, a protective film that protects the surface of the magnetic recording layer, and an underlayer of a soft-magnetic material that performs a role of concentrating a magnetic flux emitted by a magnetic head used in a recording onto the magnetic recording layer.
In order to improve the basic characteristics of a magnetic recording medium, it is necessary to improve the signal-to-noise ratio (SNR). That is, it is necessary to increase the signal output from the magnetic recording medium, and reduce the noise. One cause of a decrease in signal output and an increase in noise is an increase in the orientational dispersion (variation in crystal orientation) of the magnetic recording layer. With the perpendicular magnetic recording medium, it is necessary to orient the easy magnetization axis of the magnetic recording layer perpendicular to the medium plane, at which time, in the event that the orientational dispersion of the easy magnetization axis increases, the signal output decreases due to a decrease in the magnetic flux in the perpendicular direction. Also, results obtained from the investigations of the inventors show that, with a medium with a large orientational dispersion, the magnetic isolation between crystal particles decreases, the magnetic cluster size increases, and the noise increases (refer to Non-patent Document 1: Shunji Takenoiri, Yasushi Sakai, Kazuo Enomoto, Sadayuki Watanabe, Hiroyuki Uwazumi, “Development and Issues of CoPtCr—SiO2 Perpendicular Recording Media”, material from 135th Research Conference on Magnetics (Mar. 12, 2004)).
Also, there has been proposed a perpendicular magnetic recording medium wherein a two-layer underlayer of a Fe, Cr, or Co alloy and Ru is disposed between the magnetic recording layer and soft-magnetic underlayer with the object of improving the magnetic characteristics, and improving the electromagnetic conversion characteristics by a decrease in the noise caused by the soft-magnetic underlayer (refer to JP-A-2002-100030). Also, in order to achieve the object, there has been proposed a perpendicular magnetic recording medium wherein a soft-magnetic underlayer formed from a CoFe alloy, and a Ru underlayer between the magnetic recording layer and the soft-magnetic underlayer, are disposed (refer to JP-A-2002-298323).
Also, there has been proposed a perpendicular magnetic recording medium wherein a underlayer formed from a soft-magnetic permalloy based material, and a non-magnetic intermediate layer having a comparatively large film thickness formed from Ru or a Ru based alloy, are disposed between the soft-magnetic underlayer and the magnetic recording layer, with an object of decreasing the orientational dispersion in the magnetic recording layer, reducing an initial growth layer, reducing crystal particle diameter, and the like (refer to JP-A-2002-358617 and JP-A-2003-123239). Furthermore, it has been proposed that, in the perpendicular magnetic recording medium wherein the soft-magnetic underlayer, the underlayer formed of a soft-magnetic permalloy based material, the intermediate layer of Ru or a Ru based alloy material, and the magnetic recording layer are disposed, a reduction in the film thickness of the intermediate layer, as well as an increase in the coercivity and squareness ratio of the magnetic recording layer, and an improvement in the recording signal SNR for the recording density employed to date, are realized by inserting a soft-magnetic Co layer or a soft-magnetic Co based alloy layer between the underlayer and intermediate layer (refer to JP-A-2004-288348).
Furthermore, there has been proposed a perpendicular magnetic recording medium of a configuration wherein the magnetic recording layer is divided into first and second perpendicular magnetic films, and a sub-film and non-magnetic intermediate film are inserted between the magnetic films, with an object of improving noise characteristics and thermal fluctuation tolerance (refer to JP-A-2001-101643). In this configuration, the first and second perpendicular magnetic films are magnetically coupled. The object of the configuration is to prevent a magnetization fluctuation of the second perpendicular magnetic film by making the magnetic anisotropic energy of the first perpendicular magnetic film, which is the lower layer, greater than the magnetic anisotropic energy of the second perpendicular magnetic film, which is the upper layer, and to reduce noise by making the recording domain boundaries of the second perpendicular magnetic film linear. Also, with this configuration, it is possible to improve the thermal fluctuation tolerance by employing the first perpendicular magnetic film with the larger perpendicular magnetic anisotropic energy.
Also, there has been proposed a perpendicular magnetic recording medium wherein a first underlayer, a first non-magnetic intermediate layer, a second underlayer, and a second non-magnetic intermediate layer are disposed between the soft-magnetic underlayer and the magnetic recording layer, the first underlayer being formed of a material having an fcc structure including at least Ni and Fe, and the second underlayer being formed of a soft-magnetic material having an fcc structure including at least Co, with an object of improving the noise characteristics and SNR (refer to JP-A-2008-117506). With this configuration, by providing the stacked structure of the first non-magnetic intermediate layer, the second underlayer, and the second non-magnetic intermediate layer, the crystal growth of each of these layers is curbed, and the crystal particle diameter of each layer is miniaturized. As a result of this, the effect of the crystal particle diameter miniaturization provided by the first underlayer formed of a material having an fcc structure including at least Ni and Fe is utilized in the miniaturization of the crystal particle diameter of the magnetic recording layer.
However, with the aim of yet higher recording density, there still exists a demand for a perpendicular magnetic recording medium with which it is possible to realize a high signal output and low noise, and achieve a high SNR, even at a time of a high density recording.
In order to realize a high SNR by increasing the signal output and reducing the noise of a perpendicular magnetic recording medium, it is necessary to make the orientational dispersion of the magnetic recording layer as small as possible.
In addition to the above-mentioned point, it is necessary to reduce the crystal particle diameter of the magnetic recording layer in order to lower the noise of the magnetic recording medium. This is because, in the event that the crystal particle diameter of the magnetic recording layer increases, the bit transition region becomes irregular, and transition noise increases. Consequently, it is necessary to decrease the transition noise by reducing the crystal particle diameter and making the bit transition region linear. With regard to this point, it is known that the underlayer or intermediate layer has a function of controlling the crystallinity, orientation, crystal particle diameter, and the like, of the magnetic recording layer formed on the underlayer or intermediate layer, and affects the characteristics of the magnetic recording layer. In particular, when forming the magnetic recording layer on the underlayer or intermediate layer using epitaxial growth, the crystal particle diameter of the magnetic recording layer complies with the crystal particle diameter of the material of the underlayer or intermediate layer. Consequently, in order to reduce the crystal particle diameter of the magnetic recording layer, it is effective to reduce the crystal particle diameter of the underlayer or intermediate layer.
Furthermore, from the point of view of improving the recording density of the perpendicular magnetic recording medium, it is necessary to reduce the noise in the bit transition region. In order to do this, it is effective to secure a precipitous recording magnetic field, and make the transition as linear as possible. Herein, in order to obtain a precipitous recording magnetic field, it is necessary to make the distance between the soft-magnetic underlayer and the magnetic head as small as possible. Also, as the recording magnetic field of the magnetic head decreases as the recording density increases, it is also necessary to reduce the distance between the soft-magnetic underlayer and the magnetic head in order to secure a sufficient recording magnetic field. Generally, a non-magnetic underlayer and/or intermediate layer is provided between the magnetic recording layer and soft-magnetic underlayer. However, at present, the non-magnetic underlayer and/or intermediate layer has a film thickness of around 20 to 30 nm, and this large film thickness is a cause of increasing the distance between the soft-magnetic underlayer and the magnetic head. Actually, in the configurations presently proposed as heretofore described, the non-magnetic underlayer and/or intermediate layer has a large film thickness (for example, 35 nm or more in the configurations described in JP-A-2002-100030 and JP-A-2002-298323), and is insufficient with regard to reducing the distance between the magnetic head and the soft-magnetic underlayer, and obtaining a high SNR at a time of a high density recording.
However, it is known that, when reducing the film thickness of the underlayer or intermediate layer, a decrease in the crystal orientation of the magnetic recording layer material and a deterioration of magnetic isolation between magnetic crystal particles occur, and the magnetic characteristics of the magnetic recording layer decrease. Considering the above point, it is necessary, rather than simply reducing the film thickness, to carry out a reduction of the film thickness of the underlayer or intermediate layer while maintaining or improving the magnetic characteristics of the magnetic recording layer.